combination of several variables in driver
It's really pretty simple. You hit 'Add Variable' several times, pick the sources you want, name the variables in their top most fields more intelligently than me and then use the exact same names in the expression.

In this example, I just add the Y values of the latin gang together and drive Bombo's Y axis with them.

1.055 + 1.138 + 0.906 = 3.099